<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <script>
    let user = '....';
    let username;

    user = null; // null数据类型 释放存储空间（内存） 
    console.log(username); // undefined数据类型 定义但未赋值

    



  </script>
</body>

</html>